Superlattice structures, in general, are known and typically comprise a composite structure made from alternating ultrathin layers of different materials so that alternating barrier layers and quantum well layers are provided. Typically, the superlattice has an energy band structure which is different than, but related to, the energy band structure of the component materials.
Superlattices have many uses, including, but not limited to, use in the field of thermoelectric materials. Such materials may be used for various application including heating, cooling, power generation and temperature sensing. Whall and Parker have suggested using a superlattice prepared by molecular beam epitaxy (MBE) to enhance the thermoelectric figure of merit of a thermoelectric material. They expressed particular interest in silicon and SiGe alloys, but also mentioned the possibility of using PbTe, InAs and the transition silicides (e.g. CoSi.sub.2). Specifically, a strained-layer superlattice consisting of 20 or more layers of Si and SiGe composition was suggested.
The use of MBE to grow superlattice structures (e.g., GaAs and AlAs) is disclosed in U.S. Pat. No. 4,261,771 issued to Dingle, et al. As disclosed in Dingle et al., the general technique for fabricating a superlattice by MBE comprises the following standard steps: (1) obtaining a substrate (e.g., from a commercial source); (2) cleaning a major surface of the substrate using standard preparation procedures; (3) placing the substrate in an evacuable metal chamber; (4) reducing the chamber pressure; (5) loading shuttered effusion cells (ovens) with the requisite source materials for growth; (6) heating the substrate to about 600 degrees C., to cause desorption of contaminants from the growth surface and then adjusting the substrate temperature to that desired for growth; (7) with the shutters closed, heating the ovens until the source materials vaporize; and (8) opening selected shutters to effect growth until the desired layer thickness is attained on the substrate.
According to Dingle et al., the fabrication process can be described as forming a new composition of matter of A (e.g., GaAs) and B (e.g., AlAs or Ge) by directing a periodically pulsed molecular beam at a substrate. During the first part of each period an A-beam is directed at the substrate for a time effective to grow material A having a thickness of n monolayers and during the second part of each period directing a B-beam at the substrate for a time effective to grow material B having a thickness of m monolayers.
Therefore, the fabrication of a superlattice by MBE, or other epitaxial growth techniques, is generally known. However, the materials and their relative amounts that make up the superlattice and substrate are predominant factors in determining the characteristics of the superlattice. For use as a thermoelectric material, it is desirable to choose the materials that make up the superlattice, and their relative amounts, so that the thermoelectric figure of merit is maximized.
The thermoelectric figure of merit (ZT) is a measure of the effectiveness of a material and is related to material properties by the following equation: EQU ZT=S.sup.2 .sigma.T/.kappa. (1)
where S, .sigma., .kappa., and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and temperature, respectively. The Seebeck coefficient (S) is a measure of how readily electrons (or holes) can change energy in a temperature gradient as they move across a thermoelectric element, and is related to the strength of interaction of charge carriers with the lattice and the available energy states. The highest useful Seebeck coefficients are found in semiconductor materials with low crystal symmetry. In theory, to maximize ZT, one would try to maximize S, .sigma. and T and minimize .kappa.. However, in practice, this is not so simple. For example, as a material is doped to increase its electrical conductivity (.sigma.), bandfilling tends to lower S and the electron component .kappa..sub.e of .kappa. increases. In most materials, ZT is maximized at doping levels approaching 10.sup.19 cm.sup.-3. Since increasing (or decreasing) one parameter may adversely increase (or decrease) another parameter, it is important to select carefully the component materials to provide a high ZT.
The figure of merit ZT is related to the thermoelectric materials factor (b*) where: EQU b*=.mu.m.sup.*3/2 /.kappa..sub.L ; (2)
where .mu. is the carrier mobility, m* is the density of states effective mass and .kappa..sub.L is the lattice thermal conductivity. The precise relationship between b* and ZT is complex. However, for simplicity, it may be approximated as follows. If it is assumed that a**=b*T.sup.5/2 and that there is one-band conduction, then ZT increases monotonically as a** increases.
A superlattice provides the opportunity to enhance ZT for a number of reasons. For example, it is known that the Seebeck coefficient increases as the period of a quasi-two-dimensional superlattice decreases. The electrical conductivity may be enhanced by means of modulation doping, which has been shown to increase the carrier mobilities in Si/SiGe strained-layer superlattices. Furthermore, the lattice thermal conductivity of a small or short-period superlattice structure is expected to be substantially lower than the average of the component materials because of augmented Umklapp phonon-phonon scattering process (in which the total phonon vector is not conserved, but rather changes by 2 .pi. times the reciprocal lattice vector) due to phonon-interface scattering effects.
Multistage thermoelectric modules, which are used, for example, to cool different types of infrared imaging arrays (and for a number of other well known applications), are well known. Typically, however, they are limited to cold-sink temperatures greater than 160 K. Thermoelectric materials, for example, Bi.sub.2 Te.sub.3 and BiSb were researched 30 to 40 years ago. Unfortunately, the best currently known production thermoelectric material, Bi.sub.2 Te.sub.3, is not capable of efficient heat removal below approximately 180 K and has a ZT less than 1.
The use of various wide-bandgap semiconductor materials such as EuTe to form an electrical insulator is well known. Also, placing a buffer layer on a substrate to aid with various types of thin film layer growth on the surface of the substrate is a well known technique.